Heat transfer control of a backward-facing step flow in a duct by means of miniature electromagnetic actuators

Flow and thermal control downstream of a backward-facing step has been performed in order to achieve heat transfer enhancement by introducing small disturbance with electromagnetic flap actuators on the step edge. Flap oscillation frequency and amplitude were both changed variously under the laminar flow condition. As the flap oscillation frequency increases, heat deterioration area just behind the step rapidly decreases its size until the oscillation Strouhal number Sr equals up to 0.35, but increases once and then intensively decreases again. Thus, the largest heat transfer enhancement is obtained at the highest oscillation frequency Sr=4.0 within the studied frequency ranges, though sub-optimum frequency around 0.2<Sr<0.35 is also obtained. From velocity measurements by PIV, it was found that intensifications of two different fluid motions, i.e., a large-scale unsteady vortex and a downward high-speed flow, are the main cause to enhance the heat transfer in the sub-optimum and in the high-frequency conditions, respectively.